gdalgorithms-list

RE: [Algorithms] View Frustum Culling

[Tom F. <to...@mu...>]

Plain text please - some people here are on non-HTML systems.

I reckon that "either" is a good answer, as long as you first cull by the
bounding sphere, which is slightly bigger (OK, a lot bigger in some cases)
but usually much faster.

So, cull by the bounding sphere, and that spots 90% of the cull cases. So
the remaining 10% are the ones you need to use AABB/OBB to spot.

And using hierarchial volumes also helps enormously on applicable shapes -
and you should probably do sphere-testing all the way down to the nodes
themselves before trying anything fancier, i.e. never do AABB/OBB testing on
a non-leaf. Much quicker in most cases to just recurse down & sphere-test
the kids.

Tom Forsyth - Muckyfoot bloke.
Whizzing and pasting and pooting through the day. 

-----Original Message-----
From: Tim Johnston [mailto:ti...@gn...]
Sent: 07 September 2000 11:50
To: gda...@li...
Subject: [Algorithms] View Frustum Culling


 
What is the best method for coarse bounding box frustum culling?
 
From what I can gather you either use axis aligned bound boxes (AABBs) or
oriented bounding boxes (OBBs), with OBBs having the advantage of tighter
fitting but need extra computations for the culling process. Also which
culling method is best, perspective transform the BB and the view frustum to
the perspective coordinate system (which with AABBs can result in testing
two AABBs which is just 6 comparisons, however you have to transform all 8
vertices of the AABB) or testing the BB against the 6 planes of the view
frustum (which seems much simpler and straightforward if that is all that is
needed.)
 
So is comparing AABBs against the frustum planes the best way to go and is
it just find the equations and test the vertices or is there some more
transforming needed in there?
 
Also what are the extra steps needed to use OBBs instead?
 
And finally a nice easy question, what is the right way to get the 6
equations of the planes if you happen to have an OpenGL matrix?
 
Any help would be great
thanks,
 
Tim.

Re: [Algorithms] FIST optimization

[Pierre T. <p.t...@wa...>]

> Bet your Renderer->Init() function winds up loading a DLL somewhere along
> the line (for example, by initialising D3D). It's the DLL load that is
> probably whacking your FPU state.

I checked: it gets modified by the CreateDevice call. I suppose it could be
the DLL loading, yes.


> You should set the FPU state to the one you want after you do your Init().
> Thereafter, D3D won't mess with it if you set the FPU_PRESERVE flag.

That's what I do. That was exactly my question on the DXML : is that safe ?

Because:
- D3D assumes a round-to-nearest mode
- I'm changing it to chop/floor
- And as I said, FPU_PRESERVE seems only to work with the FPU precision (not
the rounding mode).

Hence the original question. Anyway, I already use your solution, and for
the moment I don't really need something else.

Pierre

Re: [Algorithms] rather curious

[<ro...@do...>]

Jeff Lander  wrote:

>Man, we are all working late tonight.  On a sunday, that's kinda sad.

Nah,  I spent a pleasurable day listening to a free concert by the San
Francisco Opera in Golden Gate Park, and am so wiped out by sunburn
that sitting in front of the tube and pontificating on physics is an
entirely appropriate activity.

>But to drag this back to algorithms, your use of Newton's gravitation law is getting you into trouble.  
>

>Realize that the only thing that effects when two dropped objects hit the ground
> is the acceleration of those objects.  Force doesn't enter into it yet.  

Actually, in most developments of the logical foundations of physics
that I've seen I think that  "force" isnot well defined.  In the way I
was using it, in terms of the "four forces",  it refers to a type of
"interaction".

>....
>So you can see that a body of any mass will hit the earth at the same time.  
>The difference due to the original gravitation law is that a more massive body
> will hit the earth with more FORCE and make a bigger dent.
>

Here, I think your use of "FORCE" is not appropriate.  The two bodies
fall under the same acceleration, so fall for the same time duration
and hit the earth with the same speed, but the more massive body has
more kinetic energy and more momentum, so makes a bigger dent. 

RE: [Algorithms] FIST optimization

[Herb M. <HMa...@En...>]

Anything loaded in your process space can change your FPU mode.

Simply reset your FPU control word to a known/standard state before you
start your collision routines.

You can use _asm, or in VC++ use _control87

- herb

Ensemble Studios
http://www.ensemblestudios.com

mailto:hma...@en...
ICQ:35921824

Age of Empires=AE




-----Original Message-----
From: John Ratcliff [mailto:=09
Sent: Saturday, September 09, 2000 12:28 PM
To: gda...@li...
Subject: [Algorithms] FIST optimization


As many of you already know, the call to 'ftol' can be one of the =
biggest
hotspot spikes in a 3d application.  Every time a floating point number =
is
converted into an integer, rather than simply producing a 'fist'
instruction, MSVC will instead invoke a very slow procedure call to =
'ftol'.
I have heard this is to ensure that the rounding behaviour of the =
operation
conforms to ANSI standard or something like that.

Back when I was an assembly programmer I just used the fist instruction
wherever needed.  With MSVC there is a compiler optimization switch =
called
/Qifist, which allows MSVC to use the 'fist' instruction instead of the =
call
to 'ftol'.

When I enable this optimization my performance profile is greatly =
improved.
At the heart of my collision detection routines is a hash, where I take =
the
floating point x,y co-ordinate in world space and remap it into an =
index to
this precomputed collision detection table.  This is a critical =
operation
that requires the conversion from floating point to int, very =
efficiently.

However, the behavior I am experiencing is as if the rounding mode were
occasionally set to something other than truncate behavior.  Meaning, =
where
I expect a conversion from 2.99999 to int to produce a 2, not a 3.  The
behaviour is spurious and unpredictable.  When I worked in assembly =
code I
never had this problem.

My main question is, how can the rounding mode ever get changed?  Or am =
I
missunderstanding the behavior possibly?

The symptom is as follows.  Without the /Qifist option enabled my =
collision
detection works flawlessly.  With the /Qifist enabled it behaves =
spuriously,
exactly as if sometimes it rounds the float to hash index incorrectly, =
thus
accessing the wrong pre-computed collision detection polygon set.

Why can't you guarentee that the rounding mode is set to truncate =
throughout
the application?  I could switch to inline assembly I suppose, but I =
prefer
to have the compiler generate the code naturally with all of it's =
normal
optimizations enabled.  Does anyone know the preferred method to =
guarentee
the processor is in a rounding mode which causes truncation to always =
occur?
Or am I missinterpreting the problem entirely?

Thanks,

John

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RE: [Algorithms] Game Developer Salary

[Scott, J. <JS...@ra...>]

As we seem to be going even more OT....

Waitress: Nice accent, are you from Australia?
Me: Nope, England.
Waitress: Almost the same, I mean you used to own Australia didn't you?

Waitress: Are you from England?
Me: Yes.
Waitress: I have a friend in New England... etc.

Ordering at Taco Hell is a challenge and a half.

Back on topic - the big PC game developers are all in the States. I got sick
of learning a new platform every project. The standard of living is much
higher here, I have no regrets about moving. My biggest fears were the
violence (media hype; I'd like to take a few Yanks into a few English towns
at kicking out time), and the fizzy pop called 'beer' over here (there's
Guinness and Newcastle everywhere).

Salaries are always compensation for your job; it is a balance between how
much you enojoy working where you do and how much money they pay. I
personally would rather be paid a reasonable amount and love the games I
work on with the people at the company, than being paid extra to work on
sports sims with a bunch of people I don't like.

Cheers
John

-----Original Message-----
From: Jonathan Wight [mailto:jw...@bi...]

on 9/11/00 8:46 AM, Stephen J Baker at sj...@li... wrote:

> Honestly, I could tell one of these stories for every six months I've
> lived here in Sunny Texas.

My favourite is when I when I buy something via mail order over the phone
and _always_ (without fail) get the salesman/woman (usually the latter) who
"has always wanted to visit England" or "has a friend who just got back from
England". There must be something about my accent that makes people want to
discuss their vacation plans  over the phone with me.

> Well, you can negotiate more holiday (I did) - and I'm earning
> probably 2x what I could in UK for similar cost of living.

Same here, although I'm not in the games industry so can't comment on the
salary ratios there. I'd say cost of living here in Austin, TX is a lot less
than back home as well.
 
>> Sounds very much like slave trade to me.
> 
> In some respects, it is.  You certainly don't have *freedom* for
> the period while you are waiting for the INS department to process
> your GreenCard paperwork

Luckily for me my wife is American and I'm not working under some kind of
indentured servitude. Friends of mine who have worked under other visas have
really rough times. In fact one person I know worked for free for a period
(this was a person with a Masters in CompSci and some industry experience)
just to get sponsorship.

    Jon.

RE: [Algorithms] FW: [CsMain] Scene Graphs

[Tom F. <to...@mu...>]

Dead. Toast. Finished. Pushing up dasies. This is an ex-API.

They just haven't admitted it yet.

Tom Forsyth - Muckyfoot bloke.
Whizzing and pasting and pooting through the day.

> -----Original Message-----
> From: Jonathan Wight [mailto:jw...@bi...]
> Sent: 06 September 2000 22:46
> To: gda...@li...
> Subject: Re: [Algorithms] FW: [CsMain] Scene Graphs
> 
> 
> on 9/6/00 1:36 PM, Ko, Manchor at MAN...@ca... wrote:
> 
> > Now there isn't. But you should be familiar with Performer 
> and Fahrenheit as
> > a start. Otherwise you are in a huge hole. I have a lot of 
> experience
> > designing scene-graphs. Will be happy to discuss it with u. 
> I have a class
> > structure that works very well.
> 
> What is happening with Fahrenheit BTW? Is it dead? I found a news link
> yesterday that seemed to say it was alive and doing well but 
> I couldn't
> confirm it.
> 
> Also has anyone got any good links to some Fahrenheit technical
> documentation?
> 
>     Jon.

RE: [Algorithms] VIPM With T&L - what about roam

[Tom F. <to...@mu...>]

Yes. Though ROAM, in other peoples' experience, is limited by the ROAM
algorithm running on the CPU, not by a T&L graphics card. So your real
bottleneck is the CPU.

Tom Forsyth - Muckyfoot bloke.
Whizzing and pasting and pooting through the day.

> -----Original Message-----
> From: Sam Kuhn [mailto:sa...@ip...]
> Sent: 06 September 2000 21:48
> To: gda...@li...
> Subject: RE: [Algorithms] VIPM With T&L - what about roam
> 
> 
> 
> 
> Ok cool, thats what I hoped.
> 
> This brings me to another thought...
> 
> Say you had a roam implementation that subdivided down 
> (ignoring camera
> distance, just using landscape variance) to a fixed level say 500,000
> vertices or so and stored them on card for T&L. *then* on 
> subsequent passes
> of the roam bintree, generate indexed lists into that 
> hardware vertex pool
> based on the camera distance. This means you get true 
> distance based roam
> LOD and cached t&l of the used verticies right?
> 
> sam

Re: [Algorithms] Game Developer Salary

[Jonathan W. <jw...@bi...>]

on 9/11/00 8:46 AM, Stephen J Baker at sj...@li... wrote:

> Honestly, I could tell one of these stories for every six months I've
> lived here in Sunny Texas.

My favourite is when I when I buy something via mail order over the phone
and _always_ (without fail) get the salesman/woman (usually the latter) who
"has always wanted to visit England" or "has a friend who just got back from
England". There must be something about my accent that makes people want to
discuss their vacation plans  over the phone with me.

> Well, you can negotiate more holiday (I did) - and I'm earning
> probably 2x what I could in UK for similar cost of living.

Same here, although I'm not in the games industry so can't comment on the
salary ratios there. I'd say cost of living here in Austin, TX is a lot less
than back home as well.
 
>> Sounds very much like slave trade to me.
> 
> In some respects, it is.  You certainly don't have *freedom* for
> the period while you are waiting for the INS department to process
> your GreenCard paperwork

Luckily for me my wife is American and I'm not working under some kind of
indentured servitude. Friends of mine who have worked under other visas have
really rough times. In fact one person I know worked for free for a period
(this was a person with a Masters in CompSci and some industry experience)
just to get sponsorship.

    Jon.

RE: [Algorithms] Game Developer Salary

[Stephen J B. <sj...@li...>]

On Mon, 11 Sep 2000, Stefan Boberg wrote:

> > Brits are pretty good as they speak almost the same
> > language.
> 
>   "Almost the same language"... erhhhh!?? :)
> 
>   Reminds me of a friend who went to work for Digital Domain... One day in
> the shops this woman asks him where he's from - "Scotland" he says, "Oh
> really! But you speak such good English!".

...or at a Jack-in-the-Box drive thru counter:

Her:   "I love your accent - where are you from?"  (something I hear
                    daily as a Brit living in Texas)
Me:    "England".
Her:   "Oh - so what language do they speak there?"

...or at a Dairy Queen:

Her:   "I love your accent - where are you from?"
Me:    "England".
Her:   "Oh...That must be a LONG drive."

...or...

Honestly, I could tell one of these stories for every six months I've
lived here in Sunny Texas.

>   This is precisely why I opted not to go and work in the US. It's
> ridiculous. That and the fact that you only get 2wks paid holiday (seems to
> be standard, anyway). Sure you can get paid lots more (in theory, anyway),
> but I prefer the freedom I have within the EU.

Well, you can negotiate more holiday (I did) - and I'm earning
probably 2x what I could in UK for similar cost of living.

>   Sounds very much like slave trade to me.
 
In some respects, it is.  You certainly don't have *freedom* for
the period while you are waiting for the INS department to process
your GreenCard paperwork (Geological processes work faster) - but
then the wages are very good and the cost of living low (although
*not* if you live in California).

----
Science being insufficient - neither ancient protein species deficient.

Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
L3Com/Link Simulation & Training (817)619-2466 (Fax)
Work: sj...@li...           http://www.link.com
Home: sjb...@ai...       http://web2.airmail.net/sjbaker1

RE: [Algorithms] Game Developer Salary

[Scott, J. <JS...@ra...>]

>  "Almost the same language"... erhhhh!?? :)
>
>  Reminds me of a friend who went to work for Digital Domain... One day in
>the shops this woman asks him where he's from - "Scotland" he says, "Oh
>really! But you speak such good English!".

You'd be amazed at how much trouble I have getting myself understood
sometimes. Webster also decided to misspell some words, which has caused a
few problems. eg. the secondary attack in Heretic2 is "+defence". I didn't
know at the time =)

>  This is precisely why I opted not to go and work in the US. It's
>ridiculous. That and the fact that you only get 2wks paid holiday (seems to
>be standard, anyway). Sure you can get paid lots more (in theory, anyway),
>but I prefer the freedom I have within the EU.

It's effectively double the salary and half the taxes of a similar job in
the UK. When does anyone get time to go on holiday anyway ? =)

Cheers
John

RE: [Algorithms] Game Developer Salary

[Stefan B. <ste...@te...>]

> As an aside, if you want someone to stay loyal to your company,
> get a non US citizen. Brits are pretty good as they speak almost the same
> language.

  "Almost the same language"... erhhhh!?? :)

  Reminds me of a friend who went to work for Digital Domain... One day in
the shops this woman asks him where he's from - "Scotland" he says, "Oh
really! But you speak such good English!".

> An employee under a visa is not going to move for anything less than a
factor
> of 2 salary increase (or extreme unhappiness) because it is such a pain to
> get or renew a visa.

  This is precisely why I opted not to go and work in the US. It's
ridiculous. That and the fact that you only get 2wks paid holiday (seems to
be standard, anyway). Sure you can get paid lots more (in theory, anyway),
but I prefer the freedom I have within the EU.

> They are also tied to the company, if you sack them,
> they have to return back to their country of origin.

  Sounds very much like slave trade to me.

Cheers,
Stef
--
Stefan Boberg, R&D/Technical Manager, Team17 Software Ltd.
bo...@te...

RE: [Algorithms] Game Developer Salary

[Stephen J B. <sj...@li...>]

On Mon, 11 Sep 2000, Scott, John wrote:

> As an aside, if you want someone to stay loyal to your company, get a non US
> citizen. Brits are pretty good as they speak almost the same language. An
> employee under a visa is not going to move for anything less than a factor
> of 2 salary increase (or extreme unhappiness) because it is such a pain to
> get or renew a visa. They are also tied to the company, if you sack them,
> they have to return back to their country of origin.

Grrrr, grumble...complain.

Advice to any Brit who takes up this kind of a deal: Make sure you
get a good salary at day #1 because you won't see a pay raise until
you get your green card - and in some parts of the US, that's taking
6 years...also you are chained to that job - you really can't leave.

----
Science being insufficient - neither ancient protein species deficient.

Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
L3Com/Link Simulation & Training (817)619-2466 (Fax)
Work: sj...@li...           http://www.link.com
Home: sjb...@ai...       http://web2.airmail.net/sjbaker1

RE: [Algorithms] Game Developer Salary

[Scott, J. <JS...@ra...>]

As an aside, if you want someone to stay loyal to your company, get a non US
citizen. Brits are pretty good as they speak almost the same language. An
employee under a visa is not going to move for anything less than a factor
of 2 salary increase (or extreme unhappiness) because it is such a pain to
get or renew a visa. They are also tied to the company, if you sack them,
they have to return back to their country of origin.

Cheers
John 

-----Original Message-----
From: Gavan Hood [mailto:gh...@si...]

I would have to agree with you Scott, I have noticed that my salary in the
US is higher than that quoted for Game companies, and we have the same level
of complexity and challenges. I have also noticed varied reasons for low
salaries, outside the Game industry it is often due to people that see quick
one off apps made in RAD languages (VB etc) as being more bang for buck even
though they spend hours trying to finish and fine the app which is where
they would reaped huge benifits if they used a talented guy and a flexible
language such as C /C++.

In the Game Industry I have witnessed an attitude that IT companies also
posses being, we are a cool company, our product is cool the best will
sacrafice salary just be associated with us, sure that might work for a
short period with a few people trying to upgrade thier skills but
experienced guys just dodge them for obvious reasons.

I wish I knew a solution, but I usually end up just steering clear of those
who will not pay, hoping that one day I will find a couple of other
experiences guys (10+ years expertise...) and make our own product, I think
lots of people have that hope :-)

Gavan

Re: [Algorithms] Physic : impulse computation

[Neal T. <ne...@ps...>]

Christian BRUNO <br...@if...> writes:

>I have a question on impulse computation for the particular case where one
>of the colliding objets has an infinite mass. I mean, this object cannot
>change its position or orientation.

>In the case where 2 "moveable" objects collide, i know how to compute the
>+F
>and -F forces that will prevent inter-penetration, but these forces apply
to
>both objects, and change the position and orientation of my "fixed, static"
>object.

>To prevent the fixed object from moving, or rotating, is it "physically
>correct" to apply 2*F to the only moveable object and a null force to the
>fixed object ?

>any help greatly appreciated

The general form for calculating an impulse for a frictionless collision
between two rigid bodies is:

j = -(1 + e) vRel / (A + B + C + D)

where

A = 1 / Ma

B = 1 / Mb

C = n Dot ((Inverse(Ia) x (Ra cross n)) cross Ra)

D = n Dot ((Inverse(Ib) x (Rb cross n)) cross Rb)

Here Inverse is the matrix inverse operation, dot is vector dot product,
cross is vector cross product, and x is matrix by vector transform.

In the numerator, e is the coefficient of restitution and vRel is the
relative velocity of the point of contact (see below).

In the various terms of the denominator, n is the normal at the point of
contact, Ra is the position of the point of contact relative to the centre
of mass for body a, Rb is the position of the point of contact relative to
the centre of mass for body b, Ma and Mb are the masses of bodies a and b
respectively, and Ia and Ib are the inertia tensors of bodies a and b in the
space in which you are working (usually world space).

For a collision when we know body b is immoveable, we can assume that Mb is
infinity and Inverse(Ib) is the zero matrix in whatever space we are
working.  Thus we have:

A = 1/Ma

B = 0

C = n Dot ((Inverse(Ia) x (Ra cross n)) cross Ra)

D=0

and we can calculate the relative velocity VRel for this case using

vRel = n Dot (vLinRel + (OmegaRel cross Ra))

where Dot and cross are defined as above, + is vector addition, vLinRel is
the relative linear velocity of the two bodies, Omega is the angular
velocity of body a, and Ra is the position of the point of contact relative
to the centre of mass of a.

You can then calculate the collision impulse j using these expressions for
the numerator and denominator, and apply it to the linear and angular
momenta of body a, using:

MomA = MomA + j

(since the impulse represents an instantaneous change in momentum)

and

AngMomA = AngMomA + (j cross Ra)

where j, Ra, cross and + are defined as above, and MomA and AngMomA are the
linear and angular momenta of body a.

Body b, since it has an infinite mass and a zero inverse inertia tensor, is
not affected by the impact in any way.

Hope that was some use, anyway... (and I hope I didn't make a mistake typing
out all the equations:-) - but hopefully the principle is reasonably clear
even if I've lost a sign somewhere).

Neal Tringham (Sick Puppies)

ne...@ps...
ne...@em...

RE: [Algorithms] FIST optimization

[Tony C. <to...@mi...>]

>Then I thought about the recent mails here. I then changed the way I
>initialize Direct3D, and used DDSCL_FPUPRESERVE instead of DDSCL_FPUSETUP.
>Nothing changed.
>
>...but the guilty one is D3D anyway...
>
>When I reset the FPU to the "chop" rounding mode before calling
>Renderer->Init(), it still does not work.
>
>When I reset the FPU to the "chop" rounding mode after Renderer->Init(), it
>*does* work.

Bet your Renderer->Init() function winds up loading a DLL somewhere along
the line (for example, by initialising D3D). It's the DLL load that is
probably whacking your FPU state.

You should set the FPU state to the one you want after you do your Init().
Thereafter, D3D won't mess with it if you set the FPU_PRESERVE flag.

Tony Cox - DirectX Luminary
Windows Gaming Developer Relations Group
http://msdn.microsoft.com/directx 

[Algorithms] Physic : impulse computation

[Christian B. <br...@if...>]

Hello,

[warning, frenchglish message follows...]

I have a question on impulse computation for the particular case where one
of the colliding objets has an infinite mass. I mean, this object cannot
change its position or orientation.

In the case where 2 "moveable" objects collide, i know how to compute the +F
and -F forces that will prevent inter-penetration, but these forces apply to
both objects, and change the position and orientation of my "fixed, static"
object.

To prevent the fixed object from moving, or rotating, is it "physically
correct" to apply 2*F to the only moveable object and a null force to the
fixed object ?

any help greatly appreciated
Christian


 
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RE: [Algorithms] rather curious

[Jeff L. <je...@di...>]

Man, we are all working late tonight.  On a sunday, that's kinda sad.
But to drag this back to algorithms, your use of Newton's gravitation law is getting you into trouble.  

Realize that the only thing that effects when two dropped objects hit the ground is the acceleration of those objects.  Force doesn't enter into it yet.  Here is why:

Gravity is as you say F = (Gm1*m2)/r^2, also thanks to Newton we have F=ma.

Solve for the acceleration using those two equations:

m1a =  (Gm1*m2)/r^2     // note the m in f=ma is for the dropped body which I am calling m1.

a = Gm2/r^2

The acceleration of body 1 is only dependent on the gravitation constant, distance, and the mass of the earth (or whatever).  On most of the surface of the earth as you know that a = 9.8 m/s^2.  I guess at airplane height, it is a bit different.

So you can see that a body of any mass will hit the earth at the same time.  The difference due to the original gravitation law is that a more massive body will hit the earth with more FORCE and make a bigger dent.

Of course this ignores the fact that the formulas need to be applied to the earth as it is attracted to the other object.  If the object being dropped was very massive, it would move the Earth also.  This would change the time of collision.  Of course for most objects you will ever be able to drop, this doesn't matter.

-Jeff

At 01:21 AM 9/11/2000 -0500, you wrote:
>But,
>what i brought up is that if we go by Newton's law of universal gravitation
>there is a *difference*
>
> >All bodies, in a vacuum, fall at the same rate-
>if we take this formula
>F = G ((m1*m2)/r^2)
>
>and we run this for 2 separate objects with *very similar* masses,
>F will be different.
>
>F = ma;
>a = F/m;
>
>even at a *tiny precision* they still fall at different rates, thus hitting
>the surface at a different time.
>hence i feel that this should be taught in schools at least for a
>"technical" viewpoint cause it's the *right thing* to do.

RE: [Algorithms] rather curious

[Akbar A. <sye...@ea...>]

>Nah, this is NOT true according to Einsteinian General Relativity, and
>no one has ever done an experiment sensitive enough to detect that.
>All bodies, in a vacuum, fall at the same rate--the result of
>Galileo's Tower of Pisa experiment, and a cornerstone of Einstein's
>theory, in which it is called "the Principle of Equivalence".

i'm currently doing some reading on General Relativity, <Physics for
Scientists and Engineers, by Raymond A. Serway, 3rd edition. Copyright is
pretty old>

But,
what i brought up is that if we go by Newton's law of universal gravitation
there is a *difference*

>All bodies, in a vacuum, fall at the same rate-
if we take this formula
F = G ((m1*m2)/r^2)

and we run this for 2 separate objects with *very similar* masses,
F will be different.

F = ma;
a = F/m;

even at a *tiny precision* they still fall at different rates, thus hitting
the surface at a different time.
hence i feel that this should be taught in schools at least for a
"technical" viewpoint cause it's the *right thing* to do.

*before you write anything read below ;)

>But in the several dozen experiments where GR predicts a
>measurable difference from the predictions of Newtonian gravitational
>theory, GR has been confirmed.

with the knowledge from above i see what *you mean*, but i still have to
read those books to *understand* and *see the differences* ;)
as well as to understand the essence and derivation of these theory's.


i have always thought adding any 2 arbitrary numbers will always give the
output of the sum of those 2 numbers.
i guess maybe in this level these "elementary" rules do not apply (note: i'm
a  making parallel here)?




*******
*******
this is "non-technical" and is based entirely on my ways/experience with the
"World(Tm)".
also a reply from Ron about switching career field to physics.
this is very biased :)

please DO NOT send reply's to the list about *this* unless you feel it
pertains to "gaining "technical" knowledge" " ;)
*******
*******


or maybe this is the same case with the definition of "light" (
"it's a wave, wait a sec, in this case it's acting like a particle"
here is a conversation with a truthful researcher and on of the chairs on
the book community.

my question is, why can't they just come up with a "new" definition/name,
which combines both of the properties and merges them into one?


btw, speaking of the universe and all, and somebody mentioned string theory,
faster then light issues, etc..
http://www.superstringtheory.com/basics.html
haven't read through it yet, but it "looks" somewhat legit mate.
we all know that looks can be deceiving though.




>Don't be teed off--change your career and study physics.
;)

a.) academia is not my cup of tea ( i have change my ways ;), i have *no*
idea how some of you manage to stay in school for +6 years. i guess if you
like it...

b.) to get a *decent* (imho gov jobs don't count<pay to low>) job let alone
doing research/getting paid in that field you have to have a highly
appraised *background*, and *just* going to oxford doesn't cut it.

c.) don't get me started on thesis's and some issues with proffeesor/deen
pride...

d.) i like the way carmack summarized this up (full time physics people),
the people in that field have to wait a long time to get there ideas built
up into a reality.
some times they wait years for the "authorization" to happen. then there is
organizing the team, building the facilty if it doesn't exsist, etc..

for us in the cg/real time field, we think of something and do it the next
minute :)


e.) yes, writing missile tracking software is *very* cool, this is physics
and just about every other problem in "real life at work" (Tm)!
but the restraints your stuck with are mind blowing.
imagine waking up every morning to restrains like, ("your code segment is
WAY to big, the MEMORY think of the MEMORY!") and your trying your best...

i wonder what types of hardware(memory,cpu) there running for the new missle
guidance systems.
hehe.

i really admire those guys, as well as console developers who manage to
squeeze technically cool stuff through there hardware limitaions...
a good example would be the software team which was responsible for porting
Resident Evil 2 to Nintendo 64.
see gdmag (www.gdmag.com)  for more details.
there might be an article up about it at www.gamasutra.com ?


peace,
akbar A.

"imho, and i'm sure in yours as well, this entire universe follows out a set
of equations...
the objects of course "

// couldn't fit this anywhere else ;)






-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of Ron
Levine
Sent: Sunday, September 10, 2000 10:51 PM
To: gda...@li...
Subject: Re: [Algorithms] rather curious


Akbar A.  wrote:

>hey,
>i am just wondering, but how many people know about *this*
>"gravity isn't really a force" and that it is caused by the curvature in
the
>space time fabric.
>

Of course, this thread is entirely off topic with respect to game
algorithms in the ordinary sense (unless of course you want to make
relativistic physics the object of the game :-)).  But physics is one
of the best subjects to be "rather curious" about, so no one has
objected so far.

In the modern physics paradigm,  gravity is one of the four basic
forces,  and,  by many orders of magnitude,  the weakest of the four.
The other three are called (in order of increasing strength), the weak
force (which, for example,  is responsible for the decay of neutrons
into protons, electrons and neutrinos), the electromagnetic force
(which is responsible for all of chemistry, optics and all other
electromagnetic interactions), and the strong force, (which is
responsible for the binding of quarks into protons and neutrons, and
the binding of protons and neutrons into atomic nuclei.  There has
recently been a buzz about some experimenters claiming to have
discovered a fifth force, but it has not yet entered into the
mainstream physics paradigm.  So, in terms of modern physics it is
perfectly OK to call gravity a force.

But in this paradigm, gravity is indeed also understood as a
phenomenon of the curvature of the space-time "fabric", necessarily
caused by the presence of energy (including mass).  You can completely
understand the orbits of bodies under the gravitational force as
motion along geodesics, i.e. paths of shortest length, in the curved
space-time.

The mathematics that you need to put this all on a sound
logical basis is higher-dimensional differential geometry and tensor
analysis.
This is the framework of Einstein's General Theory of
Relativity.

For the most part, for most ordinary experimental
observations, General Relativity predicts the same behavior as the
classical Newtonian gravitational model, up to the precision of
experiments.

But in the several dozen experiments where GR predicts a
measurable difference from the predictions of Newtonian gravitational
theory, GR has been confirmed.  Read any popular book on modern
physics, such as Stephen Hawking's "A Brief History of Time" for a
non-mathematical review of these ideas, experiments and astronomical
observations.  If you want the real stuff, the best book is
"Gravitation", by Misner, Thorne, and Wheeler, but it is daunting.

>And, if you drop two objects in a vacuum they *really* don't fall
>at the same rate (difference is very small, order of 10^-24) <this *makes
>sense*>

Nah, this is NOT true according to Einsteinian General Relativity, and
no one has ever done an experiment sensitive enough to detect that.
All bodies, in a vacuum, fall at the same rate--the result of
Galileo's Tower of Pisa experiment, and a cornerstone of Einstein's
theory, in which it is called "the Principle of Equivalence".

However, the main unsolved problem of today's physics is the "Theory
of Everything", which would put all four of the forces together into
one logical framework.  The three strongest forces all coexist nicely
in a theoretical framework called "The Standard Model", whose
framework is essentially quantum mechanical.  No one has yet got a
decent theory of quantum gravity, nor done any experiments that would
expose the quantum mechanical nature of gravity.  But everyone (in
physics) believes there ought to be such a theory that would unify all
the forces.  The present dominant line of theoretical investigation is
called "Superstring Theory", which claims, among other  things, that
the dimension of space-time might be as high as ten.  But superstring
theory has not yet made any predictions that are subject to
experimental verification under present experimental technology.  I
don't know but mabe you or someone has heard that some of the results
of string theory predict a departure from the Principle of
Equivalence, but I haven't heard of it, and certainly no one has
observed it in experiments.  For a good popular review of the present
state of this knowledge see the book "The Elegant Universe" by Brian
Greene.  "A Brief History of Time" was an enormous best seller in the
80's and 90's.  Greene's book was a best seller last year.

>
>i am just curious what is the *the few on this list*  knowledge about
>this...
>
>i recently (3 minutes ago) got in an argument with a friend on this
*topic*,
>he told me his professor said that "Gravity is a FORCE" and saying anything
>different from that is pure anarchy.
>
>i told him what is above is true and André Lamothe writes this in all of
his
>books, and i'm pretty sure he knows more than your professor (the laws make
>sense as well), ...

>i am probably just t-ed of cause i didn't have more proof about the space
>time fabric one :|
>

Don't be teed off--change your career and study physics. The proof is
in the elegance and economy of the theory and the fact that it has
been completely confirmed to within the precision of all experimental
tests so far.

>btw, andre, if your not on the list;
>i highly suggest you join :) i know your busy with volume 2 and all..

This comment reminds me of the sign in the post office:  "No dogs
allowed, except seeing-eye dogs".

_______________________________________________
GDAlgorithms-list mailing list
GDA...@li...
http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list

Re: [Algorithms] rather curious

[Alex M. <am...@bw...>]

----- Original Message -----
From: "Ron Levine" <ro...@do...>
To: <gda...@li...>

> expose the quantum mechanical nature of gravity.  But everyone (in
> physics) believes there ought to be such a theory that would unify all
> the forces.  The present dominant line of theoretical investigation is
> called "Superstring Theory", which claims, among other  things, that
> the dimension of space-time might be as high as ten.  But superstring

    Ummm, hrmmm, correct me if I am wrong, but the number ten is based off
of KSV (Kikkawa, Sakita, Virasoro) diagrams, which produce modular
functions, that just seem to spit ten back in extremely bizarre places.
Also, noting that ten is not the only number, as 26 has also been proven to
work for superstring theory.... but I digress....

> observed it in experiments.  For a good popular review of the present
> state of this knowledge see the book "The Elegant Universe" by Brian
> Greene.  "A Brief History of Time" was an enormous best seller in the
> 80's and 90's.  Greene's book was a best seller last year.

    I would also read, for the laymen, Hyperspace by Michio Kaku. Simple,
elegant, and my cat can understand it.


> Don't be teed off--change your career and study physics. The proof is
> in the elegance and economy of the theory and the fact that it has
> been completely confirmed to within the precision of all experimental
> tests so far.
>
> >btw, andre, if your not on the list;
> >i highly suggest you join :) i know your busy with volume 2 and all..

    Yeah, only on chapter 8, and he made another (edited I believe) in the
meantime.

_________________________
> GDAlgorithms-list mailing list
         /.\
          |
          |
 Whoa, where did the word physics go....


ERROR: Your beta-version of Life1.0 has expired. Please upgrade to the full
version. All important social functions will be disabled from now on.

Re: [Algorithms] rather curious

[Mark W. <mwa...@to...>]

> equivalent -- remember E = mc^2) causes the space surrounding it to become

Actually, correct me if I'm wrong, E = mc^2 + Eo

Sorry, couldn't help but get pedantic considering the current thread :)

Mark.

Re: [Algorithms] rather curious

[<ro...@do...>]

Joe Silver  wrote:

>Not that I have anything to add to this conversation..  but I did want to
>ask.. where is a good place or a good book to get started with physics?  my
>physics knowledge is pretty low..
>

The three-volume set "Lectures in Physics", by Richard Feynman, is
terrific.  It is pitched to the level of freshman and sophomore
science majors.  

Re: [Algorithms] rather curious

[<ro...@do...>]

Jeff Lander wrote

> At JPL, good old Newton does the job for most things.

I'm sure that this is true for _most_ things at JPL, certainly for all
the work they do on dynamics of Earth-launched space craft and their
motion within the solar system.  But not all things.

I think that at JPL there are probably also astronomers who are
involved in observing such things as the cosmic evidence for
gravitational lenses (see the spectacular photos published just last
yea--I'm not sure where, but I saw them on the NY Times Science pages)
and evidence for black holes at the centers of galaxies, and these are
definitely General Relativistic phenomena,  explainable only in the
context of space-time curvature. 

Re: [Algorithms] rather curious

[<ro...@do...>]

Robert Dibley  wrote:

>> hey,
>> i am just wondering, but how many people know about *this*
>> "gravity isn't really a force" and that it is caused by the 
>> curvature in the space time fabric.
>
>What is it then ?
>I mean being caused by the curvature of space-time doesn't explain what it
>is.

Wrong, it certainly explains very well what it is.  But you need a
little higher math to see that.

>A ball rolls down a hill because of gravity, not because of the curvature of
>the surface its on.

Right.  It's not the curvature of the surface it's on, rather it's the
curvature of the space-time continuum.

Re: [Algorithms] rather curious

[<ro...@do...>]

Akbar A.  wrote:

>hey,
>i am just wondering, but how many people know about *this*
>"gravity isn't really a force" and that it is caused by the curvature in the
>space time fabric.
>

Of course, this thread is entirely off topic with respect to game
algorithms in the ordinary sense (unless of course you want to make
relativistic physics the object of the game :-)).  But physics is one
of the best subjects to be "rather curious" about, so no one has
objected so far.

In the modern physics paradigm,  gravity is one of the four basic
forces,  and,  by many orders of magnitude,  the weakest of the four.
The other three are called (in order of increasing strength), the weak
force (which, for example,  is responsible for the decay of neutrons
into protons, electrons and neutrinos), the electromagnetic force
(which is responsible for all of chemistry, optics and all other
electromagnetic interactions), and the strong force, (which is
responsible for the binding of quarks into protons and neutrons, and
the binding of protons and neutrons into atomic nuclei.  There has
recently been a buzz about some experimenters claiming to have
discovered a fifth force, but it has not yet entered into the
mainstream physics paradigm.  So, in terms of modern physics it is
perfectly OK to call gravity a force.

But in this paradigm, gravity is indeed also understood as a
phenomenon of the curvature of the space-time "fabric", necessarily
caused by the presence of energy (including mass).  You can completely
understand the orbits of bodies under the gravitational force as
motion along geodesics, i.e. paths of shortest length, in the curved
space-time.  The mathematics that you need to put this all on a sound
logical basis is higher-dimensional differential geometry and tensor
analysis.  This is the framework of Einstein's General Theory of
Relativity.  For the most part, for most ordinary experimental
observations, General Relativity predicts the same behavior as the
classical Newtonian gravitational model, up to the precision of
experiments.  But in the several dozen experiments where GR predicts a
measurable difference from the predictions of Newtonian gravitational
theory, GR has been confirmed.  Read any popular book on modern
physics, such as Stephen Hawking's "A Brief History of Time" for a
non-mathematical review of these ideas, experiments and astronomical
observations.  If you want the real stuff, the best book is
"Gravitation", by Misner, Thorne, and Wheeler, but it is daunting. 
  
>And, if you drop two objects in a vacuum they *really* don't fall
>at the same rate (difference is very small, order of 10^-24) <this *makes
>sense*>

Nah, this is NOT true according to Einsteinian General Relativity, and
no one has ever done an experiment sensitive enough to detect that.
All bodies, in a vacuum, fall at the same rate--the result of
Galileo's Tower of Pisa experiment, and a cornerstone of Einstein's
theory, in which it is called "the Principle of Equivalence".

However, the main unsolved problem of today's physics is the "Theory
of Everything", which would put all four of the forces together into
one logical framework.  The three strongest forces all coexist nicely
in a theoretical framework called "The Standard Model", whose
framework is essentially quantum mechanical.  No one has yet got a
decent theory of quantum gravity, nor done any experiments that would
expose the quantum mechanical nature of gravity.  But everyone (in
physics) believes there ought to be such a theory that would unify all
the forces.  The present dominant line of theoretical investigation is
called "Superstring Theory", which claims, among other  things, that
the dimension of space-time might be as high as ten.  But superstring
theory has not yet made any predictions that are subject to
experimental verification under present experimental technology.  I
don't know but mabe you or someone has heard that some of the results
of string theory predict a departure from the Principle of
Equivalence, but I haven't heard of it, and certainly no one has
observed it in experiments.  For a good popular review of the present
state of this knowledge see the book "The Elegant Universe" by Brian
Greene.  "A Brief History of Time" was an enormous best seller in the
80's and 90's.  Greene's book was a best seller last year.

>
>i am just curious what is the *the few on this list*  knowledge about
>this...
>
>i recently (3 minutes ago) got in an argument with a friend on this *topic*,
>he told me his professor said that "Gravity is a FORCE" and saying anything
>different from that is pure anarchy.
>
>i told him what is above is true and André Lamothe writes this in all of his
>books, and i'm pretty sure he knows more than your professor (the laws make
>sense as well), ...

>i am probably just t-ed of cause i didn't have more proof about the space
>time fabric one :|
>

Don't be teed off--change your career and study physics. The proof is
in the elegance and economy of the theory and the fact that it has
been completely confirmed to within the precision of all experimental
tests so far.

>btw, andre, if your not on the list;
>i highly suggest you join :) i know your busy with volume 2 and all..

This comment reminds me of the sign in the post office:  "No dogs
allowed, except seeing-eye dogs".

RE: [Algorithms] rather curious

[Frag_ D. <fra...@ho...>]

    Don't have much time right now, but has anybody on this list heard of 
the string theory?  If I recall correctly, it said that there were several 
more dimensions (I think we have like eight, but I can only name the three + 
time).  Anyway, it said something that all matter was created of this 
"strings", including quirks, protons, neutrons--everything, and that the 
dimensions are so "small" that they hardly effect us.  The theory 
(theoretically, actually hypothetically--it hasn't been proven yet) explains 
the spatial warping through these new "micro dimensions", or whatever you 
want to call them, for one because it's easier to do stuff in more 
dimensions.  Consider the example:

    Two objects wish to swap places in a one dimensional environment.  
Assuming that they couldn't pass through each other, it would be impossible. 
  In two dimensions, however, one could go over and the other under.  In 
three dimensions then, you could just step around.  In four I'm sure a good 
example could be thought of.

     This is a horribly unorganized email, but you should check into it.  
Steven Hawking has looked into it a little, but I don't want to say any 
incorrect quotes or anything like that.  In game programming, this may be 
completely unnecessary and even detrimental.  Still interesting.


>From: "Akbar A." <sye...@ea...>
>Reply-To: gda...@li...
>To: <gda...@li...>
>Subject: RE: [Algorithms] rather curious
>Date: Sun, 10 Sep 2000 20:48:52 -0500
>
> >The reason people don't know a lot about relativistic physics is they 
>have
> >trouble with Galileo let alone Einstein.
>i don't want to sound stupid (being that i have not had *any* experience
>with Einstein's work) but wasn't most of his stuff, derived from material
>which was pretty trivial?
>
>
> >No one believed us that the objects hit at basically the same time
>regardless >of mass.  So we all started dropping stuff on the table to 
>prove
>it.
> >Pretty funny.
>been there, done that :)
>
>well you can always look at it and explain to other people that' it's a
>formula and you are just putting in data which has a very *large* gap.
>the precision is small but the difference is *still* there, so in all of
>it's actuality they *don't* fall at the same rate :)
>just cause we can't see it, doesn't mean it doesn't exsist ;)
>
>imho, it must of been a pain in the ass to convince people that we/objects
>are "attracted" to each other ;)
>
>
> >The whole what is gravity thing is pretty tough.  Claiming to have the
> >"right" answer is probably folly.
>aren't researchers still clamoring about a "true/right way" to describe
>light and all it's properties;
>"wait a second it's a wave, oh No it's a particle!" (i will stop this, i'm
>going on a tangent ;)
><grin>
>
> >Using Newton's equations for the force of gravity and f=ma
>the mind is so weird.
>maybe i saw this in a dream or something but wasn't it first published as
>-f = -ma
>?
>
> >Didn't they do the feather and rock demo on an Apollo mission as a pr
>thing?  >I seem to remember seeing a video of it.
>
>do we even have video capture devices that are that accurate to get the
>10^-24 precision, so we could "see" the diff?
>
>
>anyways,
>i think i got some reading to do about this topic so i can argue with a
>better foot hold.
>i hope this book does the trick, just put the order in.
>http://www.amazon.com/exec/obidos/ASIN/0517029618/o/qid=968634459/sr=2-1/102
>-7947677-2983366
>
>
>
>peace,
>akbar A.
>
>being in the game biz, we actually *apply* all these fancy equations most
>people just memorize for the midterm and forget about.
>
>
>
>
>-----Original Message-----
>From: gda...@li...
>[mailto:gda...@li...]On Behalf Of Jeff
>Lander
>Sent: Sunday, September 10, 2000 6:37 PM
>To: gda...@li...
>Subject: Re: [Algorithms] rather curious
>
>
>The reason people don't know a lot about relativistic physics is they have
>trouble with Galileo let alone Einstein.
>
>Funny this should come up today since last night we got into a little 
>debate
>about this at dinner with some friends.  My friend works at JPL and we were
>talking about skydiving and this came up.  No one believed us that the
>objects hit at basically the same time regardless of mass.  So we all
>started dropping stuff on the table to prove it.  Pretty funny.  It is
>always amazing how a simple "try it yourself" can surprise people.  In
>general they just believe it should be some way and never check it out.  No
>wonder Galileo ended up in jail.
>
>The whole what is gravity thing is pretty tough.  Claiming to have the
>"right" answer is probably folly.  Clearly Galileo/Newton works fine for
>most applications any human would run into.  I am no expert on 
>gravitational
>physics but from what I know, common thinking is based  on  Einstein's
>theory of General relativity.  The mass of objects warp spacetime and the
>falling is explained by objects following a straight path in space-time.
>
>Galileo went to great pains to prove that Aristotle and Plato were wrong
>about falling objects.  If he wasn't exactly right to the order of 10^-24 
>or
>whatever, I am not sure it matters.  At JPL, good old Newton does the job
>for most things.
>
>Using Newton's equations for the force of gravity and f=ma you can see how
>the mass of the falling object just drops out.  Different masses do cause
>the objects to hit with different forces but that is different from the
>time, eh.
>
>If someone wants to implement actual relativistic physics for simple rigid
>body simulations, I am sure it would be cool, but Newton is good enough for
>me :)
>
>Didn't they do the feather and rock demo on an Apollo mission as a pr 
>thing?
>I seem to remember seeing a video of it.
>
>-Jeff
>
>At 04:26 PM 9/10/2000 -0500, you wrote:
> >hey,
> >i am just wondering, but how many people know about *this*
> >"gravity isn't really a force" and that it is caused by the curvature in
>the
> >space time fabric.
> >
> >And, if you drop two objects in a vacuum they *really* don't fall
> >at the same rate (difference is very small, order of 10^-24) <this *makes
> >sense*>
> >why don't more people know this?
> >
> >even if it's at a difference 10^-24,
> >i assure you, if we had money that was this accurate, banks would 
>"require"
> >this much precision in there software :|
> >
> >i am just curious what is the *the few on this list*  knowledge about
> >this...
> >
> >i recently (3 minutes ago) got in an argument with a friend on this
>*topic*,
> >he told me his professor said that "Gravity is a FORCE" and saying 
>anything
> >different from that is pure anarchy.
> >
> >i told him what is above is true and André Lamothe writes this in all of
>his
> >books, and i'm pretty sure he knows more than your professor (the laws 
>make
> >sense as well), ...
> >
> >
> >
> >i am probably just t-ed of cause i didn't have more proof about the space
> >time fabric one :|
> >
> >
> >btw, andre, if your not on the list;
> >i highly suggest you join :) i know your busy with volume 2 and all...
> >
> >peace,
> >akbar A.
> >
> >
> >_______________________________________________
> >GDAlgorithms-list mailing list
> >GDA...@li...
> >http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list
>
>_______________________________________________
>GDAlgorithms-list mailing list
>GDA...@li...
>http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list
>
>_______________________________________________
>GDAlgorithms-list mailing list
>GDA...@li...
>http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list

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